For the measurement of transmission characteristics such as basic attenuation, frequency-dependent attenuation, frequency-dependent transit time, frequency offset, noise, phase jitter and discontinuous phase changes, harmonics factor, pulse noise, amplification fluctuations and path interruptions it is known to utilize different measuring devices respectively operating according to different, mostly standardized measuring processes. Depending on the type of coefficient to be measured, an input of the test object may have applied to it no measuring signal at all (e.g. for noise measurement), a measuring signal with a single frequency (e.g. for the measurement of amplitude fluctuations or interruptions), a measuring signal with several predetermined frequencies (e.g. for the measurement of the harmonics factor), or a measuring signal with continually or stepwise changing (wobbling) frequency (e.g. for the measurement of the frequency-dependent transit time or of the frequency-dependent attenuation), the measuring signal being modulated in a specific way in some instances. The respective coefficient is obtained from the incoming signal appearing at the output of the test object, possibly with the assistance of a reference signal generated on the receiving side.
The known measuring processes and the known circuit arrangements for their implementation have a number of disadvantages. Thus, it is necessary to employ different measuring devices for the determination of most coefficients on the basis of the different measuring processes, which is expensive and requires cumbersome handling. Especially in the measurement of transmission paths a changeover from the measurement of one coefficient to the measurement of the next coefficient requires always a talking connection between the operating personnel active at opposite ends of the path, which may lead to misunderstandings and errors.
A further disadvantage of the known processes is the relatively slow formation of the result in the measuring of frequency-dependent coefficients, caused by the waiting periods for the attainment of their steady state by the test object and the measuring device, which prevents or at least impedes an oscilloscopic representation and/or a rapid removal (compensation) of the distortion involved.
Likewise, the known processes enable only partly or not at all the simultaneous measurement and emission of some or all coefficients (e.g. group transit time and noise). Finally it appears desirable to form, besides the separate emission of the individual coefficients, a coefficient ("magnitude of overall distortion") facilitating a global yet rapid and unequivocal evaluation of the test object, such a coefficient being suitable for the convergent adjustment of equalizers or other devices positively affecting the transmission quality.
Processes have already been proposed which attempt to circumvent the aforementioned drawbacks in that the test object is energized by a reproducible pulse-type measuring signal which greatly resembles the signal occurring during actual data transmission and whose shape distortion by the test object is analyzed on the receiving side. In that case, however, it is not possible to emit coefficients such as, for example, group transit time or attenuation, in dependence upon frequency, with the known modes of visualization or oscilloscopic representation of said coefficients, nor is there any assurance of satisfactory performance when the test object generates a frequency offset.
It is furthermore disadvantageous that coefficients independent of the transmitted signal, e.g. noise, can be measured not at all or only with the measuring-signal transmitter cut off, which again constitutes a step backward.
From the magazine "Communication Designer's Digest" of June 1969, pages 51 to 53, it is known, in the use of a spectrum analyzer, to visualize modulated signals with high signal-to-noise or high carrier-to-sideband ratio with the aid of means designed to suppress the carrier present in a signal mixture to be analyzed, for the purpose of avoiding overloading, by subtracting from the signal mixture a noisefree auxiliary voltage of the same amplitude and phase as the carrier.